The present invention relates to a method of producing flexible polyurethane foam having reduced discoloration which utilizes low molecular weight alkanes or mixtures thereof as auxiliary blowing agents. More particularly, the present invention relates to the use of auxiliary blowing agents which are linear or branched alkanes having boiling points above -50.degree. C. and below 100.degree. C.
Currently, flexible polyurethane foam is the primary resilient cushioning material in commerce. It combines low density and high durability along with the comfort characteristics desired for many uses (e.g., furniture and bedding applications).
The principle reactants for the manufacture of flexible polyurethane foam are polyol, polyisocyanate, and water. Water reacts with a portion of the polyisocyanate to form carbon dioxide which is the principal blowing agent. To prepare some flexible urethane foams having a specific softness and low density, an auxiliary blowing agent is sometimes required. Such auxiliary blowing agents are characterized as relatively inert liquids at room temperature and which have low boiling points.
The premier auxiliary blowing agent has previously been trichlorofluoromethane, also known as CFC-11. However, regulatory pressures due to the recognition of the possibility of significant ozone depletion in the upper atmosphere have caused essentially a phase-out of the use of CFC-11 and have led to a search for alternatives.
In addition, discoloration of polyurethane foam slabs has been a chronic industry problem. The problem has been attributed to the high exotherm of the foaming reaction. Contributions to the resolution of the discoloration problem have come from, for example, addition of and improvement to inhibitors in the polyols and changes to additive flame retardant chemical structures. However, for certain polyurethane foam grades, particularly those utilizing relatively high levels of reactant water, and relatively low levels of auxiliary blowing agent, the foam discoloration problem has remained.
The search for alternatives to CFC-11 has shown that other auxiliary blowing agents are useful, with the leading candidates being dichloromethane and 1,1,1-trichloroethane. Unfortunately, the use of dichloromethane and 1,1,1-trichloroethane as auxiliary blowing agents has also been seriously questioned. The use of dichloromethane has been questioned because of health concerns while the use of 1,1,1-trichloroethane has been questioned because of the degree of upper atmosphere ozone depletion.
In our search for alternative auxiliary blowing agents, we have found several liquids which demonstrate initial feasibility. These alternatives include ketones, esters, and alkanes which have limited reactivity with the foaming ingredients of polyurethane yet have sufficiently low boiling points to allow gas formation and density reduction of the polyurethane foam. Considerable prior art exists in the preparation of flexible urethane foam utilizing hydrocarbon blowing agents. U.S. Pat. No. 3,506,600 discloses the use of lower molecular weight alkanes and mixtures thereof as foaming agents for polyurethane foam. Similar discloses are given in U.S. Pat. Nos. 3,650,993; 4,048,102, 4,317,889; 4,334,031; 4,546,122; and 4,785,027. However, in none of these patents has there been the reported use nor recognition of the specific benefits of the hydrocarbon auxiliary blowing agents for reduced discoloration.